Vietnam has extended an Indian oil concession in the South China Sea and begun drilling in another area it disputes with China in moves that could heighten tensions over who owns what in the vital maritime region.The moves come at a delicate time in Beijing's relations with Vietnam, which claims parts of the sea, and India, which recently sent warships to monitor the Malacca Straits, through which most of China's energy supplies and trade passes.Vietnam granted Indian oil firm ONGC Videsh a two-year extension to explore oil block 128 in a letter that arrived earlier this week, the state-run company's managing director Narendra K. Verma told Reuters.Part of that block is in the U-shaped 'nine-dash line' which marks the vast area that China claims in the sea, a route for more than $5 trillion in trade each year in which the Philippines, Brunei, Malaysia and Taiwan also have claims.